Since the introduction of insulin in the 1920's, continuous strides have been made to improve the treatment of diabetes mellitus. Major advances have been made in insulin purity and availability with the development of recombinant DNA technology. Various formulations with different time-actions have also been developed. Currently, there are generally seven commercially available insulin formulations: Regular insulin; semilente insulin, globin insulin, isophane insulin, insulin zinc suspension, protamine zinc insulin, and Ultralente insulin.
Despite the array of formulations available, subcutaneous injection therapy still falls short of providing a patient with convenient regulation and normalized glycemic control. Frequent excursions from normal glycemia levels over a patient's lifetime lead to hyper-or hypoglycemia, and long term complications including retinopathy, neuropathy, nephropathy, and micro- and macroangiopathy.
To help avoid extreme glycemic levels, diabetics often practice multiple injection therapy whereby insulin is administered with each meal. However, this therapy has not yet been optimized. The most rapid-acting insulin commercially available peaks too late after injection and lasts too long to optimally control glucose levels. Therefore, considerable effort has been devoted to create insulin formulations and insulin analog formulations that alter the kinetics of the subcutaneous absorption process.
Because all commercial pharmaceutical formulations of insulin contain insulin in the self-associated state and predominately in the hexamer form, it is believed that the rate-limiting step for the absorption of insulin from the subcutaneous injection depot to the bloodstream is the dissociation of the self-aggregated insulin hexamer. Recently, monomeric insulin analogs have been developed that are less prone to association to higher molecular weight forms than human insulin. This lack of self-association is due to modifications in the amino acid sequence of human insulin that decrease association by primarily disrupting the formation of dimers. See, e.g., Brems et al., Protein Engineering, 5:6, 527-533 (1992) and Brange et al., Nature, 333:679-682 (1988). Accordingly, monomeric insulin analogs possess a comparatively more rapid onset of activity while retaining the biological activity of native human insulin. These insulin analogs provide a rapid absorption to place injection time and peak action of insulin into closer proximity with postprandial glucose excursion associated in the response to a meal.
The physical properties and characteristics of monomeric analogs are not analogous to insulin. For example, Brems et al. disclose that various monomeric analogs have little, or no, Zn-induced association. Any association that is observed is to a multitude of higher molecular weight forms. This differs dramatically from insulin, which is almost exclusively in an ordered, hexamer conformation in the presence of zinc. Brange et al. Diabetes Care 13:923-954 (1990). The lack of association attributes to the fast acting characteristics of the analogs. Because the analogs have lower tendency to associate, it is quite surprising that a monomeric insulin analog can be formulated to provide an intermediate duration of action.
The present invention provides a monomeric insulin analog formulation that yields upon use an intermediate duration of action. The invention further provides a novel protamine crystal called insulin analog-NPD. The present invention also provides a mixture of insulin analog-NPD and soluble monomeric insulin analog. This mixture provides a rapid onset of action and an intermediate duration of action. Accordingly, the mixture possesses advantages over both insulin and the monomeric analog. The present invention further provides for a process for preparing uniform crystals of insulin analog-NPD.